Merit of Ge and Na(Tl) detectors as determined by imaging efficiency and scatter rejection

Deep-lying cold lesions are often of marginal object contrast. Any contrast reduction caused by the detector may hinder their detectability. The capabilities of Ge and NaI(Tl) detectors are compared on the basis of imaging efficiency and scatter rejection capability. When the NaI(Tl) baseline is at 126 ke V (for 99Tcm), the void contrast obtainable with Ge detectors is 50% higher than that with NaI(Tl) using collimators having equally high resolution. If the signal-to-noise ratio (S/N) with Ge is not equal to or higher than that with NaI(Tl), this contrast advantage can be masked by excessive statistical noise. Since the lower detection efficiency of Ge crystals largely offsets their higher contrast, to obtain equal S/N in equal imaging times the Ge detection area must be comparable to that of NaI(Tl). When the NaI(Tl) baseline is at 145 keV, the window efficiency is reduced but the contrast is nearly as high as that with Ge. The Ge detection area for equal S/N is then only 40% of the NaI(Tl) area. Thus, a Ge detector having only half the area of NaI(Tl) is of questionable merit. Ge detector arrays comparable in area to NaI(Tl) are feasible and would clearly be superior imaging devices.